Re: [win-pv-devel] xenstore/wmi behaviour

[James Dingwall <james@xxxxxxxxxxxxxx>]

On Fri, Nov 06, 2015 at 05:04:16PM +0000, Paul Durrant wrote:
> > I am accessing xenstore using python via wmi and I appear to have
> > inconsistent behaviour compared to Linux with xenstore-read.  My
> > xenstore key is:
> > 
> > /a/path/to/a/key = ""
> > 
> > In xs.c in the xen source:
> > 
> >             char *val = xs_read(xsh, xth, argv[optind], &len);
> >             if (val == NULL) {
> >                 warnx("couldn't read path %s", argv[optind]);
> >                 return 1;
> >             }
> > 
> > So a NULL return value is used to indicate that the key does not exist.  
> > Testing
> > this:
> > 
> > # xenstore-read /a/path/to/a/key ; echo $?
> > 
> > 0
> > # # no output or error
> > 
> > However reading the same key on Windows through wmi instead results in
> > python getting a None type object where I believe it should
> > get an empty string "".
> > 
> > Setting up a connection in Python through wmi to xenstore then:
> > 
> > >>> print session.GetValue("/a/path/to/a/key")
> > (None,)
> > 
> > In powershell:
> > PS C:\>$base = gwmi -n root\wmi -cl XenProjectXenStoreBase
> > PS C:\>$sid = $base.AddSession("MyNewSession")
> > PS C:\>$session = gwmi -n root\wmi -q "select * from
> > XenProjectXenStoreSession where SessionId=$($sid.SessionId)"
> > PS C:\>$v = $session.GetValue("/a/path/to/a/key").value
> > PS C:\>echo $v.getType();
> > You cannot call a method on a null-valued expression.
> > At line:1 char:1
> > + echo $v.getType()
> > + ~~~~~~~~~~~~~~~~~
> >     + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
> >     + FullyQualifiedErrorId : InvokeMethodOnNull
> > PS C:\>echo "".getType()
> > 
> > IsPublic IsSerial Name                                     BaseType
> > -------- -------- ----                                     --------
> > True     True     String                                   System.Object
> > 
> > So the behaviour outside of python seems to be the same.
> > 
> > I have tried to follow the xen source code and I didn't find any special 
> > NULL -
> > > "" conversion in the read path but it is quite
> > possible I have missed something.  Therefore I believe that
> > SessionExecuteGetValue() of wmi.c should be doing something special so
> > there is compatibility in its behaviour between operating systems.
> > 
> 
> Yes, there should be a clear distinction between an empty value and a 
> non-existent value so it does seem this is a bug. Thanks for pointing it out.
> 
>   Cheers,
> 
>     Paul
> 
> > Thanks,
> > James

Hi,

We have been testing a recent build of the pv-drivers and have noted that this 
issue is still present.  Is it 
something that is likely to be an easy fix which we can test?  We have a common 
python module for working with 
xenstore in Linux and Windows and I would prefer to not have to code around 
this issue if possible.  Also is 
there a bug tracker for the pv drivers project where I can report this?

Thanks,
James

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